SUMMARY Skeletal muscle has remarkable capacity for regeneration following injury. This capability derives from resident muscle stem cells, known as satellite cells (SCs), which reside between the myofiber and its surrounding basal lamina. During normal homeostatic maintenance of adult muscle, SCs are quiescent. Following injury, they are ?activated? to generate the myoblasts required for regeneration and self-renew to replenish the SC pool. Stem cells, including SCs, reside in a specialized microenvironment, or ?niche?, that supports their behavior, as demand requires. Niche-dependent maintenance of SC quiescence is of particular importance as breaking quiescence in SCs generally leads to depletion of the SC pool and impaired regenerative capability. SCs may be useful in cell-based therapies for muscular dystrophies and myopathies, or for age-related muscle loss. Alternatively, it has been proposed that targeting stem cell niches themselves may enhance the therapeutic potential of endogenous stem cell populations. Therefore, a thorough understanding of the SC niche is both fundamental to muscle biology and critical for harnessing SCs for therapeutic purposes. A major constituent of the quiescence-inducing SC niche is the myofiber itself, but little is known of the molecular components that mediate this phenomenon. We have now identified the cell-cell adhesion molecules N-cadherin (Ncad) and M- cadherin (Mcad) as quiescence-promoting factors of the SC niche. SCs in mice lacking Ncad/Mcad (dKO mice) break quiescence in vivo, but unlike most other instances in which SC quiescence is broken, loss of these cadherins results in a stable increase in SC numbers that persists for at least one year. Furthermore, dKO mice repair both single and triple injuries similarly to control mice. Strikingly, dKO mice replenish their SC numbers to the same elevated level they had prior to injury, even after the triple injury. Loss of SC quiescence in dKO mice is therefore associated with an apparently stable expansion of the SC pool. dKO SCs may be in a partial state of activation; this is associated with an incomplete perturbation of cadherin-based adhesive structures that includes nuclear translocation of a fraction of SC ?-catenin. Our findings raise important questions about the nature of the adhesive niche itself and how it promotes SC quiescence. To gain further insight into these processes, the following aims are proposed: 1. Determine the nature of the cadherin-based adhesive SC niche and the consequences of cadherin loss in SCs on mdx mice; and 2. Determine the roles of the catenin proteins as niche factors in SC adhesion, quiescence, and activation. SCs have potential therapeutic value and have served as an enormously valuable stem cell model system. Cell-autonomous properties of SCs are much better understood than is niche-dependent regulation of SC behavior. The experiments proposed in this grant application seek to address this gap with reference to cadherins as SC niche factors. Successful completion of the proposed aims will provide highly significant information for muscle and SC biology generally and in relation to the ability to exploit SCs in a therapeutic context.